The present invention relates to a method for controlling a doubly-fed machine.
A doubly-fed machine is an electric machine, in which both a stator and a rotor can be fed with voltage. Most typically the doubly-fed machines are connected such that a stator winding of the machine is connected directly to a supplying network or a network to be supplied, and a rotor winding is connected to the same network through a controllable device, such as a cycloconverter or a frequency converter. Thus, the stator windings are directly affected by the network voltage, whereas rotor magnetization can be modified in a suitable manner.
The doubly-fed machine is generally used in applications with high nominal powers. When employed as a generator, typical applications include wind generators. In that case the generator is controllable in the vicinity of the nominal speed range by a converter connected to a rotor circuit. This converter should be rated to process only slip power in connection with the control. The control range achieved can be about 30% over or below the synchronous speed of the machine. The rating of the converter or the feeding devices thereof is relatively low as compared with a wide control range to be achieved, which makes the use of the doubly-fed machine an inviting alternative. Correspondingly, for the same reason in motor applications requiring high power the doubly-fed machine is an interesting alternative to consider as a motor, if said control range in the vicinity of the synchronous speed is sufficient.
According to prior art, the control of doubly-fed machines is implemented by modelling the machine as precisely as possible, and on the basis of the model an inverter is controlled to implement the targets set for the machine. This machine model is extremely complicated and includes numerous parameters that are often to be determined machine-specifically. The parameters to be determined include inductances and resistances of the machine, for instance. It should be noted that the parameter values are approximations of real quantities, which may vary in accordance with point of operation. In addition, the operation of a reliable model requires considerable computational capacity.